Ovarian cancer encompasses many molecularly and etiologically distinct diseases, but patients have not yet benefitted from our evolving understanding through the development and translation of subtype-specific molecularly targeted therapies. We hypothesize that, for a small molecularly-defined subset of ovarian cancer patients, the secreted protein protease inhibitor SPINK1 may represent a key oncogenic driver of tumor proliferation, invasion, and chemoresistance, and that SPINK1 may offer a novel therapeutic target for this group of women. We further hypothesize that these activities of SPINK1 are mediated not through protease inhibition, but through a newly discovered activity of SPINK1 as a growth factor capable of activating the epidermal growth factor receptor (EGFR). Here, we propose three specific aims designed to test these hypotheses. (1) We will define how SPINK1 affects ovarian cancer cell growth, invasion, and drug resistance, using cell culture models of ovarian cancer in combination with SPINK1 gene silencing and recombinant SPINK1 treatment. We will also test the ability of SPINK1 neutralizing antibodies to inhibit the growth an invasion of SPINK1-positive ovarian cancer cells, and to sensitize cells to chemotherapy. (2) We will define the molecular interaction between SPINK1 and EGFR, using quantitative binding measurements and X-ray crystallography. We will also determine the role of EGFR signaling in the cancer-promoting activities of SPINK1 in ovarian cancer cells. (3) We will evaluate expression of SPINK1 in a large set of invasive epithelial ovarian cancer biospecimens, testing the association with EGFR signaling pathway activation and with survival. In aggregate, this work has the potential to define SPINK1 signaling as a key driver of malignancy for a previously unrecognized subset of ovarian cancer patients, and as a molecular target for novel drug development.